1. Field of the Invention
The present invention relates generally to data transfer between a central computer (CPU) and peripheral device(s). More specifically, the present invention relates to the interconnecting channel units, or buffer units, which are configured between the CPU and the peripheral devices. More specifically, the present invention relates to a technique which enables effective data buffering of the channel unit, allowing for an extension of the distance between the CPU and the peripheral devices.
2. Description of the Prior Art
In order to improve data transfer efficiency between a central computer and a plurality of peripheral devices, it is a current practice to provide a plurality of channel units between the central computer and the corresponding peripherals. Each of the channel units carries out buffering wherein data is gathered when it is not necessarily needed, thereby making the stored data readily available when it is required.
The present invention is directed to transferring data from the central computer to the peripheral. The following discussion will focus on this aspect of the present invention.
When a channel unit receives a command to write data to the peripheral device from the central computer, the channel unit initiates a connection with the peripheral device into which data is to be written. That is, the channel unit applies an "address out" (ADO) signal and a "select out" (SLO) signal to a peripheral device controller. The channel unit is able to confirm the connection with the peripheral device when it receives an "operational in" (OPI) signal therefrom. After establishing the channel connection, the channel unit issues a data transfer request to the central computer, after which the channel unit acquires data from the central computer. At this time, the data is located in a data buffer within the channel unit.
Data acquisition by the channel unit is initiated after a connection is established between the channel unit and the peripheral device controller. Therefore, during the initial stage of the data transfer (prior to the establishment of the connection), insufficient data is stored in the buffer. In more specific terms, if the peripheral device takes the form of a high speed device, such as a hard disk, the data transfer into the buffer of the channel unit is slower than the speed of the peripheral. Thus, the peripheral is apt to be forced to wait idle during a period of time when data is accumulated in the buffer of the channel unit. Accordingly, in order to avoid such a problem, the prior art undesirably limits the distance between the central computer and the peripheral devices.